1. Field of the Invention
This invention relates to a process for the production of polycarboxylates by selective oxidation of polysaccharides with nitrogen dioxide.
2. Statement of Related Art
The production of polycarboxylates by oxidative treatment of polysaccharides, for example cellulose, starch and dextrins, is described in detail in the literature, cf. for example Houben-Weyl "Methoden der organischen Chemie", Thieme-Verlag, Stuttgart (1987), Vol. E 20, Makromolekulare Stoffe, Sub-chapter "Poly-saccharid-Derivate" by Dr. K. Engelskirchen, loc. cit., pages 2042 et seq., more particularly pages 2124 et seg. (oxidation products of cellulose) and pages 2166 et seg. (oxidized starches). Information on production processes can also be found in the publication "Cellulose Chemistry and its Applications" (1983), John Wiley & Sons, Chichester, GB, cf. in particular T. P. Nevell, "Oxidation of Cellulose" (Chapter 10) and the literature cited therein (loc. cit. pages 262 to 265).
Various oxidizing agents are used for the oxidation of polysaccharides, more particularly polyglucosans synthesized solely from glucose, and include for example (atmospheric) oxygen, hydrogen peroxide, sodium hypochlorite or bromite, periodic acid and periodates, lead(IV) acetate, nitrogen dioxide and cerium(IV) salts. The oxidizing agents react very differently with the anhydroglucose units, cf. for example the formula schemes in Houben-Weyl, loc. cit. page 2124. Thus, periodates and lead(IV) acetate promote C-C-opening of the anhydroglucose rings. So-called 2,3-dialdehyde cellulose is obtained from cellulose, dialdehyde starch being analogously obtained from starch. Where cellulose is exposed to the action of nitrogen dioxide, the oxidation of the primary alcohol group to the carboxyl group is predominant. The oxidizing agent, which is generally present in equilibrium with dinitrogen tetroxide, may be used in gaseous formed or as a solution in an inert organic solvent, cf. also Houben-Weyl, loc. cit. page 2125 and the primary literature cited in this regard therein. Substantially selective oxidations of the primary alcohol group of the anhydroglucose units to the carboxyl group can also be carried out correspondingly starting out from starch. Thus, the oxidation of starch with gaseous nitrogen dioxide or nitrogen dioxide dissolved in water or in various organic solvents at normal temperature and pressure is known from U.S. Pat. No. 2,472,590. In this case, the reaction temperature is relatively low.
Under these conditions, the substantially complete conversion of the primary alcohol groups of the polysaccharides into carboxyl groups is only achieved after very long reaction times of, in some cases, several days. In addition, large amounts of nitrogen dioxide, based on polysaccharide to be oxidized, are required in the known processes. The long reaction times are an obstacle to continuous operation and, in practice, only allow discontinuous operation with relatively small quantities of reactants. Another problem is the exothermic nature of the reaction.
The present invention seeks to improve the production of such oxidation products of polysaccharides in order to enable relatively large quantities to be economically produced and hence to secure their availability. This is because the polycarboxylates formed are potential builders or co-builders for detergents and cleaning compositions. The same also applies to the salts of such polycarboxylates, more particularly their water-soluble salts. The use of oxidized polysaccharide compounds to boost the cleaning performance of detergents and/or cleaning products has been known per se for decades and has been repeatedly investigated (cf. for example Dutch patents NL 69 883 and NL 78 087). The replacement of phosphate-based builder systems by 6-carboxycellulose treated with Lewis acids is described in U.S. Pat. No. 3,740,339 and in U.S. Pat. No. 3,790,561. The use of oxidized polysaccharide derivatives as a builder system for boosting detergency, particularly in laundry detergents, is also proposed in Dutch patent application NL 70/02 500. In this case, however, the derivatives in question are not derivatives selectively oxidized at the C.sub.6 atom, but rather oxidation products formed by splitting of the anhydroglucose units between C.sub.2 and C.sub.3. Finally, European patent application EP 425 369 describes surfactant-containing mixtures for the washing of laundry which contain a builder system of conventional phosphate compounds, zeolite and oxidation products of cellulose, starch or glucose sirup. There are no reproducible particulars in this document relating to the preparation of the oxidized saccharide compounds described therein. In addition, stabilization of the polysaccharide oxidates initially formed by catalytic hydrogenation is said to be desirable.